Navigation apparatuses for a vehicle have been known, which include an odometer for detecting the distance over which a vehicle travels, a bearing sensor for detecting the direction of progress of a vehicle, and a map storage device. Such a navigation apparatus is capable of only displaying the present position of the vehicle on the map image read from the map storage device.
FIG. 8 shows the configuration of the above-described conventional navigation apparatus for a vehicle. The navigation apparatus includes a map storage device 1, an odometer 2, a bearing sensor 3, a switch and a touch panel 4, an input interface 5, an internal bus 6, a central processing unit 7, a RAM 8, a ROM9, an output interface 10, and a CRT 11.
The map storage device 1 which stores map data may be in the form of a video disk, a video tape, CDROM or the like. Map data on a desired area in the map data stored in the map storage device is read out and displayed on the CRT 11. The odometer 2 generates distance data for unit distance. The bearing sensor 3 generally generates bearing data by detecting the geomagnetic bearing corresponding to the direction of progress of the vehicle. The distance over which the vehicle travels after it has left the starting point and the direction in which the vehicle travels or as to whether or not the vehicle turns right or left are detected by a combination of the odometer 2 and the bearing sensor 3. The CPU calculates the present position of the vehicle on a predetermined course using the measured data, and displays the obtained data on the map. A driver inputs data such as a destination, enlargement of the map and so on through the switch and touch panel 4 when the navigation is to be started so as to input the destination, the present position of the vehicle and so on in the RAM 8. The touch panel may be provided on the screen of the CRT 11. When the driver selectively touches a desired touch area in a plurality of touch areas, a corresponding signal is generated. The input interface 5 is a signal converter through which the input from the sensors, the storage device and the switches are sent to the internal bus 6. The ROM 9 stores programs for inputting/outputting the various types of data through the input interface 5, the output interface 10 and the internal bus 6, as well as those for processing the data. The programs are executed by the CPU 7.
The odometer of the type which detects the angular velocity of the wheel or the rotational speed of a transmission is known. Thus, in the conventional navigation system, the distance through which the vehicle travels is detected by detecting the angular velocity of the wheel or the rotational speed of the transmission. This method ensures an accurate detection of the travel distance, because the accurate radius of the wheel can be obtained. However, it has an disadvantage that, when a vehicle skids, e.g., when the vehicle goes round a sharp curve, is braked suddenly or travels on a graveled road, an accurate travel distance cannot be obtained. It is also a common practice to calculate the travel distance using the vehicle speed detected by the vehicle speed sensor. This method is advantageous in that the distance over which the vehicle actually travels can be obtained regardless of the skid of the vehicle. However, the maximum correlation value is calculated, and this does not ensure a very accurate detection of the travel distance as obtained by means of the odometer, increasing the measurement errors when the vehicle travels at a high speed.
These result in the erroneous detection of a present position T on a course as a present position M, as shown in FIGS. 9 and 10.
An object of the present invention is to provide a navigation apparatus which is designed to eliminate the aforementioned problems of the prior art, and which enables an accurate travel distance detection and hence an accurate present position detection.